This is a no-brainer. Adjust the towing line so the dink is surfing down your stern wake--pick a wave, any one. There is so little drag you can grasp the line with your eyelids and it won't pull free. Let out more line so it's clawing uphill, and two hands aren't enough to hang on some times. People who claim towing dinghies reduces their speed are, well, not smart.
Don't get me wrong: I don't claim that towing dinghies is smart--but sometimes it's both necessary and safe.